Cause

The underlying cause of CRD is Mycoplasma gallisepticum (Mg). The condition is frequently triggered by respiratory viruses such as ND and IB and subsequently complicated by bacterial invasion. The main bacteria found in the infection are Mycoplasma gallisepticum and E. coli. Stress caused by moving birds, by debeaking or other operations or other unfavourable conditions e.g. cold or bad ventilation, make the birds more susceptible

Transmission

The main problem is that parent birds infected with mycoplasma gallisepticum can transmit the organism through the egg to their offsprings.In addition, infection can occur by contact or by airborne either through dusts or droplets. The incubation period varies from 4 days to 3 weeks

Species affected:

Chickens and turkeys

Clinical signs

Young chickens (broiler chicks or layer pullets) will show respiratory distress. The birds frequently show a lack of appetite, decreased weight gain and increased feed conversion ratios. In adult birds the most common symptoms are sneezing, coughing and general signs of respiratory congestion.

In laying birds, a drop of egg production between 20-30% can occur. CRD does not normally cause an alarming number of deaths. The effect is more of a chronic nature causing reduced weight gain and feed conversion ratios in broilers and lower egg production in breeders and layers. In this way the overall economic loss can be very significant in broilers but less dramatic in breeders and layers.

Internal lesions

A reddish inflamed trachea and/or cheesy exudates in airsacs, expecially in complicated cases (e.g. with secondary E. coli infections) are observed. In mild Mg infections the only lesion might be slight mucous in trachea and a cloudy or light froth in the airsacs.

Diagnosis

Diagnosis of Mg infection can be made by blood testing of chickens post-mortem examination and ultimately by isolating the causative Mg organism from tracheas or air-sacs of affected birds.

Differential diagnosis
Respiratory virus infection (ND or IB) with secondary infection (E.coli, etc.)can give similar lesions.

Treatment

Treatment of Mg-infected chickens or turkeys with suitable antibiotics or chemotherapeutics has been found to be of economic value. However, control by medication or vaccination and eradication of Mg infections has been by far the most effective method of combating the disease.

Fertile eggs from infected birds can be treated with antibiotics such as Tylosin to eliminate the Mg-organisms. Methods used are the injection of fertile eggs or egg dipping. Blood serum testing of breeder chickens from Mg antibodies has become a routine to test flocks for a Mg infection